fix(GuildChannel): overload permissionsFor and BaseManager#resolve[id]

[kyranet]

Please describe the changes this PR makes and why it should be merged:

The behaviour of GuildChannel#permissionsFor, is that if it's able to resolve the data, it will always return permissions (since the private methods memberPermissions and rolePermissions never return null):

discord.js/src/structures/GuildChannel.js

Lines 113 to 119 in 9ffcd83

	permissionsFor(memberOrRole) {
	const member = this.guild.members.resolve(memberOrRole);
	if (member) return this.memberPermissions(member);
	const role = this.guild.roles.resolve(memberOrRole);
	if (role) return this.rolePermissions(role);
	return null;
	}

For data to be resolvable, it expects either an instance or a string:

discord.js/src/managers/BaseManager.js

Lines 59 to 63 in 9ffcd83

	resolve(idOrInstance) {
	if (idOrInstance instanceof this.holds) return idOrInstance;
	if (typeof idOrInstance === 'string') return this.cache.get(idOrInstance) || null;
	return null;
	}

If an instance is provided, it will always return the instance itself, therefore, when passing a value that matches Holds, the type return should always be Holds as well.

When passing a string, the return can be nullable, so this is excluded from the non-nullable return.

Things change when GuildMemberManager#resolve is used, as it's overloaded to support more types (specifically User instances), which resolves an ID and then tries to get the GuildMember from it, that is why I am not using Exclude<GuildMemberResolvable | RoleResolvable, string>.

discord.js/src/managers/GuildMemberManager.js

Lines 46 to 52 in 9ffcd83

	resolve(member) {
	const memberResolvable = super.resolve(member);
	if (memberResolvable) return memberResolvable;
	const userResolvable = this.client.users.resolveID(member);
	if (userResolvable) return super.resolve(userResolvable);
	return null;
	}

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.